Swing-type excavation machines, for example hydraulic excavators and front shovels, require significant hydraulic pressure and flow to transfer material from a dig location to a dump location. These machines direct the high-pressure fluid from an engine-driven pump through a swing motor to accelerate a work tool at the start of each swing, and then restrict the flow of fluid exiting the motor at the end of each swing to slow and stop the work tool.
The restriction placed on the flow of fluid exiting the motor at the end of each swing has historically be generated by one or more pressure relief valves. In particular, the high pressures produced during swing deceleration are typically forced through pressure relief valves to a low pressure tank, the pressure relief valves restricting the flow of fluid to a desired degree. As pressures within the system are reduced by the draining fluid, the pressure relief valves would close by greater amounts thereby increasing the restriction on the fluid until the swinging motion eventually stopped altogether.
Unfortunately, in most situations, pressure settings of the pressure relief valves cause the valves to close completely before the swinging motion of the work tool is completely stopped. This premature closing allows residual fluid left in the circuit to be pressurized by the final swinging movement. And because this fluid has a pressure just less than the pressure required to open the pressure relief valves, the fluid would not be relieved to the tank. Instead, the trapped fluid would act as a compressed spring. After the swinging motion in the first direction stops, the spring decompresses (i.e., the pressurized fluid expands back against the motor) and functions to accelerate the work tool in an opposing direction. Fluid is again trapped during this return swing, and it functions as another spring to push the work tool back in the first direction. In some situations, this operation continues multiple times, bouncing the work tool back and forth, until the energy is finally dissipated in the form of heat.
One attempt to reduce the oscillation when a hydraulically driven member of a machine is decelerating, stopping, or reversing directions, is disclosed in U.S. Pat. No. 7,296,404 of Joseph Pfaff that issued on Nov. 20, 2007 (the '404 patent). The '404 patent provides a method that controls deceleration of the hydraulically driven member. A variable filter function is configured to control the rate at which a motion command goes to zero to stop the machine member so that the command does not close a related valve providing fluid to the member faster than the machine member can respond without oscillation.
Although the system of the '404 patent may help to reduce the amount of oscillation of a hydraulically driven member that is stopping, it may be less than optimal. In particular, the '404 patent does not distinguish between situations where normal swinging movements of the member create large enough pressure differentials to trigger relief valves, and situations where operator input indicates a neutral position where pressure differentials may not be sufficient to trigger standard relief valves.
The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.